Bile is a complex secretion produced by the liver. It is stored in the gall bladder and periodically released into the small intestine to aid in digestion. Bile components include cholesterol, phospholipids, bile pigments, and various toxins that the liver eliminates through biliary/fecal exclusion. Bile salts are synthesized and actively secreted across canalicular membranes providing the osmotic force to drive the flow of bile. This is the rate-limiting step for bile formation. Bile flow is essential for liver detoxification, digestion, cholesterol metabolism, and absorption of lipid-soluble nutrients and vitamins.
Bile acids are critical as carriers for elimination of cholesterol from the body through biliary secretion and as a detergent for the ingestion of fatty acids and fat-soluble vitamins (23). Bile acids also play important roles in regulating cell apoptosis/survival (37; 38; 39; 40; 41) and in regulating gene expression through the farnesoid X-activated receptor (42; 43; 44; 45; 46; 47) in hepatocytes. Bile acids are synthesized in hepatocytes from cholesterol, secreted into the bile after being conjugated with glycine or taurine, reabsorbed in the small intestine, and recirculated back to hepatocytes through the portal vein. Canalicular secretion of bile acids from liver into the bile is a key process in the enterohepatic circulation of bile acids and its malfunction results in different hepatic diseases (23). If this process is disrupted, accumulation of bile acids often causes liver damage due to detergent effects. In humans, the bile acid pool circulates 6-10 times every 24 h, resulting in daily bile salt secretion of 20-40 g in about 400 ml (51; 49).
Common bile acids found in the bile of selected mammals include the following:
 Common NameR′R″R″′Commonly found in speciesCholic acidα-OHα-OHHbear, cat, hamster, human,(3α7α12)mouse, pig, rabbit, ratChenodeoxycholicα-OHHHbear, hamster, human, pigacid(3α7α)Deoxycholic acidHα-OHHcat, human, rabbit(3α12α)Ursodeoxycholicβ-OHHHbearacid(3α7β)Lithocholic acidHHHhuman, rat, mouse(3α)β-muricholic acidβ-OHHβ-OHmouse, rat(3α6β7β)α-muricholic acidα-OHHβ-OHpig, mouse, rat(3α6α7β)Ω-muricholic acidβ-OHHα-OHmouse, rat(3α6α7α)R′′′′-taurine or glycine
Bile Salt Export Protein (BSEP, ABCB11, or Sister of P-glycoprotein (SPGP)), a bile canalicular ATP-binding cassette (ABC) protein, has been identified as the main transport system for the biliary secretion of bile acids (50; 13). BSEP mutations in humans lead to impaired bile salt secretion and a severe liver disease, progressive familial intrahepatic cholestasis type 2 (PFIC2) (1, 2). Bile acid secretion in PFIC2 patients is usually less than 1% of normal (2). BSEP has also been implicated as being a target for drugs that cause cholestasis (3-6). BSEP mutations have also been associated with chronic intrahepatic cholestasis, benign recurrent intrahepatic cholestasis type 2 (BRIC 2) (7, 8) and Intrahepatic Cholestasis of Pregnancy (9, 10). Mouse Bsep transports bile acids in the order of preference: taurochenodeoxycholate>tauroursodeoxycholate=taurocholate>glycocholate=cholate (11-16). Rat liver plasma membrane vesicles exhibit similar preference (17). The bile acid preferences and activity of BSEP are similar among human, rat and mouse.
bsep knockout (KO) mice suffer a cholestatic illness with increased mortality in pups, decreased fertility in adults, and bile flow only ¼ of normal amounts (18). Residual bile flow in bsep KO mice is greater than that of PFIC2 patients, and the phenotype less severe, in that bsep KO mice can survive infancy and have a normal lifespan (18). The livers of bsep KO mice express elevated levels of P-glycoprotein (Mdr1a/1b) and their bile contains novel species of bile acid, including tetrahydroxylated bile acids (THBAs) not normally present in mouse or human bile (16, 18). When fed a diet of 0.5% cholate the bsep KO mice become severely cholestatic but at the same time secrete a large amount of bile salt into the bile. To explain this apparently contradictory result, a ‘rain barrel’ model was proposed, suggesting the containment level of bile salt in hepatocytes depends on both the affinity of the transporter for bile acids (Km) and the rate of bile acid output (19). The bsep KO mice exhibit severe cholestasis on a cholate-enriched diet, since their high bile flow rate and bile acid output is mediated by a transporter whose Km is not low enough to reduce accumulated intrahepatic bile salt below toxic levels. The rain barrel model predicts that the alternative bile salt transporter has a lower affinity for cholate than BSEP.
When plasma membrane vesicles from the hamster B30 cell line, containing a high level of P-glycoprotein (Mdr1, Abcb1a) were examined, ATP-dependent taurocholate transport (20) with a Km of 69 μM, about seven-fold higher than Bsep was observed, suggesting P-glycoprotein transports taurocholate with a relatively low affinity. Analysis of biliary bile salt composition in bsep KO mice indicates that P-glycoprotein favours the less hydrophobic muricholates and THBAs over the more hydrophobic primary bile acids in both human and mouse (18, 21). This may explain the differing severity of the cholestatic phenotypes in bsep KO mice and PFIC2 in humans. In the bsep KO mice, murine P-glycoprotein transports intrahepatic muricholate and THBAs, across the canalicular membranes to maintain nearly normal bile flow, resulting in a mild phenotype. Since humans do not normally synthesize muricholate or THBAs, this option is not available to human MDR1 and results in the severe cholestasis of PFIC2 where bile flow diminishes to 1% of normal (2).
Upregulated Mdr1a/1b expression (16, 20) in the bsep knockout mice, and the known functional redundancy of the ABCB/P-glycoprotein family suggested a role for Mdr1 in mediating bile flow. However, while the bsep knockout mice exhibit very mild cholestasis throughout life, mdr1a−/−/mdr1b−/− double knockout mice are healthy, with no obvious phenotype, though they do have specific defects in biliary excretion of infused drugs that are known to be Mdr1 substrates (22).
Some cholestatic conditions, such as Primary Biliary Cirrhosis, are treated by to supplementation with a low-toxicity bile acid not normally found in human bile, ursodeoxycholate. Dietary supplementation with ursodeoxycholate did not result in greater bile flow in bsep KO mice and may even have been toxic, suggesting that BSEP is responsible for the bulk of natural ursodeoxycholate transport, and so ursodeoxycholate may not help PFIC2 patients or anyone else suffering from a BSEP insufficiency, whether inherited, associated with pregnancy, or resulting from adverse drug or dietary exposures.